The present invention relates to a cooling apparatus for semiconductor chips (hereinafter referred to simply as a “cooling apparatus”). More specifically, the invention relates to a cooling apparatus that includes an insulator base board on which semiconductor chips are arranged, a metal base on the front surface of which the insulator base board is bonded, radiation fins formed on the back surface of the metal base such that the radiation fins are integrated with the metal base to form a unit, and a duct cover that surrounds the radiation fins to cause a coolant flow between the radiation fins for removing the heat generated by the semiconductor chips.
Power semiconductor chips are used in the inverter circuit of a variable-speed motor-driving apparatus in an electric vehicle control system. Since the power semiconductor chips generate heat when a large current flows therethrough, the power semiconductor chips are used in combination with a cooling apparatus. In electric vehicle control systems, in which the weight of the cooling apparatus mountable and the space for mounting the cooling apparatus, are both limited, a liquid-cooling-type cooling apparatus that uses a circulating coolant is employed for improving the heat dissipation performance (cf. the following Patent Document 1).
In the cooling apparatus of a liquid-cooling type, thin sheet-shaped radiation fins are integrated with a metal base into a unit such that the thin sheet-shaped radiation fins are arranged uniformly in the duct, in which a coolant is caused to flow. An insulator base board, on which the semiconductor chips that generate heat are arranged, is bonded to the metal base. By causing a pressurized coolant to flow in the duct, the heat energy generated from the semiconductor chips is transferred efficiently to the coolant via the radiation fins having large surface areas. The coolant, the temperature of which is raised by the heat dissipated from the semiconductor chips, is cooled in an external heat exchanger. The coolant, after being cooled, is pressurized by a pump and returned to the duct in which the radiation fins are arranged.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. Hei. 9 (1997)-283674
In the cooling apparatus as described above, the coolant temperature in the vicinity of a duct inlet port, to which the coolant cooled by the external heat exchanger is returned, is the lowest but rises as the coolant flows through the duct toward the outflow port. Accordingly, the temperature of the coolant that cools the semiconductor chips is continually elevated and is the highest in the vicinity of a duct outflow port. Therefore, in the cooling apparatus that arranges semiconductor chips along the flow direction of the coolant, the coolant temperature under the semiconductor chips becomes higher as the coolant flows downstream (as the semiconductor chip is positioned downstream on the coolant flow). Therefore, the semiconductor chip temperature becomes higher as the semiconductor chip in question is positioned downstream along the coolant flow.
If there exist temperature differences between the semiconductor chips, the output currents from the respective semiconductor chips are determined by the output current from the semiconductor chip, the temperature of which is the highest. Although the other semiconductor chips can make a larger output current flow, the output currents thereof are limited to the output current from the semiconductor chip, the temperature of which is the highest. Therefore, the other semiconductor chips tend to be undesirably limited in their output.
The temperature differences between the semiconductor chips may be eliminated by disposing many inflow ports and many outflow ports. However, the disposition of many inflow ports and outflow ports complicates the structure. To produce sufficient coolant flow it is necessary to pressurize the coolant to a high level.
In view of the foregoing, it is desirable to obviate the problems described above. It is also desirable to provide a cooling apparatus that facilitates the reduction of the temperature differences between the semiconductor chips using a simple structure.